A device of the general kind defined above is described in European published Patent Application No. EPO 073 286A, in which the torsion damping device is interposed in the drive train from the engine of the vehicle to the road wheels, upstream (i.e. on the driving side) of a variable speed drive or transmission unit of the variable pulley type. The support means or springs of the damping device is fixed to guide rings of a conventional torsion damper, with its hub being fixed with respect to the damper plate of the damper itself.
In that type of construction the resilient means operate in the atmosphere, so that they are in a polluted environment. Most commonly the components of a conventional torsion damper are used, for example those of a friction clutch.
There are severe constraints on the overall axial length available for the accommodation of this type of damping device between the wheel member, such as a flywheel, and the forward face of the variable speed transmission unit.
This is primarily because of the presence of the fastening member securing the wheel member to its shaft, and of the bearing in front of the transmission unit.
This is especially true since, because this type of damping device needs to be so arranged as to undergo large angular displacements as between the hub and its support means (in particular so as to obtain effective suppression of vibrations), there is no incentive to arrange the resilient means or springs so that they extend over a relatively large circumferential length, thereby elongating them. For large angular displacements, the longer the resilient means (springs) are made, the more they will be deformed. In other words they are compressed to a greater extent, especially by centrifugal force. The result is that the springs suffer greater wear in contact with the elements in which they are housed. This efect is increased still further by the radial movements which occur between the damper plate and the support means.